1. Field of the Invention
The present invention is directed to wireless communication systems and, more particularly, to enabling on-demand transmissions of data signals in the uplink of a communication system without explicit respective scheduling assignments.
2. Description of the Art
A communication system consists of the DownLink (DL), conveying transmissions of signals from a base station (NodeB) to User Equipments (UEs), and of the UpLink (UL), conveying transmissions of signals from UEs to the NodeB. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
The DL supports the transmission of data signals carrying the information content, control signals providing information associated with the transmission of data signals, and Reference Signals (RSs) which are also known as pilot signals. The UL also supports the transmission of data signals, control signals, and RSs.
DL data signals are conveyed through a Physical Downlink Shared CHannel (PDSCH). UL data signals are conveyed through a Physical Uplink Shared CHannel (PUSCH). DL control channels may be of broadcast or UE-specific nature. Broadcast control channels convey system information to all UEs and, depending on their transmission rate, include a Primary Broadcast CHannel (P-BCH), which conveys the Master Information Block (MIB), and Secondary Broadcast CHannels (S-BCH), which convey Secondary Information Blocks (SIBs). The SIBs are further distinguished into SIB1 and SIB-x, x>1. UE-specific control channels can be used to provide Scheduling Assignments (SAs) for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs), as well as for other purposes. The SAs are transmitted from the NodeB to respective UEs using Downlink Control Information (DCI) formats through respective Physical Downlink Control CHannels (PDCCHs). In the absence of PUSCH transmissions, a UE conveys Uplink Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH), otherwise, the UE may convey UCI together with data information through the PUSCH.
In addition to providing DL SAs or UL SAs, DCI formats may provide common DCI for multiple UEs including:
a) Transmission Power Control (TPC) commands for PUSCH or PUCCH transmissions.
b) Scheduling information for a response to Random Access CHannels (RACH) from UEs (RACH response).
c) Scheduling information for a Paging CHannel (PCH).
d) Scheduling System Information (SI) for SIB1 transmissions. An SIB-x, x>1, transmission is scheduled through SIB1 while the MIB transmission always occurs at a predetermined time and frequency position.
DCI formats providing common DCI for multiple UEs are transmitted in PDCCH resources monitored by all UEs. DCI formats providing UE-specific DCI are transmitted in UE-specific PDCCH resources.
The DL or UL Transmission Time Interval (TTI) is assumed to be one sub-frame, and 10 sub-frames constitute one frame as illustrated in FIG. 1. One sub-frame has duration of 1 millisecond (1 msec) 110 and one frame has duration of 10 msec 120. It is further assumed that the MIB transmission in the P-BCH is in sub-frame 0, the SIB1 transmission in the S-BCH is in sub-frame 5, and PCH transmissions may be in sub-frames 0, 4, 5 and 9.
FIG. 2 illustrates the coding and transmission of a DCI format by the NodeB. A CRC of (non-coded) DCI format bits 210 is first computed in block 220 and it is then masked with Radio Network Temporary Identifier (RNTI) bits 240 using an exclusive OR (XOR) operation 0 in block 230. The CRC and the RNTI are assumed to have the same size such as, for example, 16 bits. It is XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC is appended to the information bits of a DCI format in block 250, channel coding, such as convolutional coding, is performed in block 260, followed by rate matching to the allocated PDCCH resources in block 270, and finally by interleaving, modulation, and transmission of control signal 290 in block 280.
If the DCI format conveys a DL SA or an UL SA, the RNTI is a Cell-RNTI (C-RNTI). DCI formats 3/3A use the TPC-RNTI, and DCI format 1C uses the RA-RNTI for the RACH response, the P-RNTI for the PCH, and the SI-RNTI for SIB1. Then, after descrambling with the respective RNTI, a UE can determine whether a DCI format is intended for it by performing a CRC check.
The UE receiver performs the reverse operations of the NodeB transmitter to determine whether the UE has an assigned DCI format. This is illustrated in FIG. 3. A received control signal 310 corresponding to a candidate DCI format is demodulated, and the resulting bits are de-interleaved in, block 320. The rate matching applied at the NodeB transmitter is restored in block 330, and the bits are decoded in block 340. After decoding, DCI format bits 360 are obtained after extracting CRC bits in block 350, which are then de-masked at 370 by applying the XOR operation with assumed RNTI 380. Finally, the UE performs a CRC test in block 390. If the CRC test passes, the UE considers the DCI format as a valid one and may further act depending on DCI format information and the type of its RNTI. If the CRC test does not pass, the UE disregards the presumed DCI format.
A PUSCH sub-frame structure is shown in FIG. 4. A sub-frame 410 includes two slots. Each slot 420 includes seven symbols used for the transmission of data and/or control information. Each symbol 430 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. The PUSCH transmission in one slot may be in the same part or at a different part of the operating BandWidth (BW) than in the other slot and this will be referred to as Frequency Hopping (FH). Some symbols in each slot may be used for RS transmission 440 in order to provide channel estimation and enable coherent demodulation of the received data or control signal. The transmission BW is assumed to include frequency resource units, which will be referred to herein as Resource Blocks (RBs). Each RB is assumed to consist of NscRR=12 sub-carriers, also referred to as Resource Elements (REs). UEs are allocated one or more (consecutive or non-consecutive) RBs 450 for PUSCH transmission.
FIG. 5 illustrates a UE transmitter block diagram for the PUSCH. Coded data bits 510 are provided to a Discrete Fourier Transform (DFT) block 520, the REs corresponding to the assigned transmission BW are selected through subcarrier mapping in block 530 through control of localized FDMA 540. The Inverse Fast Fourier Transform (IFFT) is performed in block 550 and a CP is inserted in block 560 and filtering through time windowing is applied in block 570 for a transmitted signal 580. For brevity, additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated. Also, the encoding and modulation process for the data bits are omitted for brevity. The PUSCH signal transmission is assumed to be over clusters of contiguous REs in accordance with the DFT Spread Orthogonal Division Frequency Multiple Access (DFT-S-OFDMA) method, allowing signal transmission over one cluster 590 (also known as Single-Carrier Frequency Division Multiple Access (SC-FDMA)), or over multiple non-contiguous clusters of contiguous BW as shown by 595.
FIG. 6 illustrates a NodeB receiver block diagram for the PUSCH. After an antenna receives a Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters), which are not shown for brevity, a digital signal 610 is filtered through time windowing in block 620 and the CP is removed in block 630. Subsequently, the NodeB receiver applies a Fast Fourier Transform (FFT) in block 640, selects the REs used by the UE transmitter through subcarrier mapping in block 660 under control of reception bandwidth in block 650. An Inverse DFT (IDFT) is applied in block 670, and modulated and coded data bits 680 are obtained. As for the UE transmitter, well known NodeB receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity.
One of several possible DCI formats is referred to as DCI format 0, which is described in Table 1 through a set of Information Elements (IEs) for operating BWs of NRRUL=6/25/50/100 RBs. Additional IEs or different number of bits per IE than those in Table 1 may apply. Zero padding may be included in DCI format 0, if needed, in order to make its size equal to the size of a DL SA DCI format (DCI format 1A). DCI format 1A may be used for scheduling transmissions of PCH, RACH response, or SIB.
TABLE 1IEs for DCI Format 0 for BW of 6/25/50/100 RBs.Information ElementNumber of BitsCommentDCI Format Indication Flag1Indicate Format 0 or Format 1AResource Allocation (RA)5/9/11/13For Consecutive RBsModulation-Coding Scheme (MCS)5Up to 32 MCS LevelsNew Data Indicator (NDI)1New TB transmission (Yes/No)Transmission Power Control (TPC)2Transmission power control commandCyclic Shift Indicator (CSI)3CSI for RS transmissionFrequency Hopping (FH)1Frequency Hopping (Yes/No)CQI Request1Include CQI in PUSCH (Yes/No)Resource Allocation Type1Contiguous or Non-contiguousZero Padding1/1/1/0DCI format 0 = DCI format 1ACRC (C-RNTI)16 C-RNTI masks the CRCTOTAL37/41/43/44
The first IE provides flag differentiating between DCI Format 0 and DCI Format 1A, which are designed to have the same size.
The second IE provides Resource Allocation (RA) in RBs assuming contiguous transmission BW. For a total of NRRUL RBs, the number of possible contiguous RB allocations is 1+2+ . . . +NRBUL=NRBUL(NRBUL+1)/2 and can be signaled with ┌ log2NRDUL(NRDUL|1)/2)┐ bits; where ┌ ┐ denotes the ceiling operation which rounds a number to its next higher integer.
The third IE provides a Modulation and Coding Scheme (MCS). For example, the modulation may be QPSK, QAM16, or QAM64, while the coding rate may take discrete values between 1/16 and 1.
The fourth IE is the New Data Indicator (NDI). If the NDI IE is set to 1, a new Transport Block (TB) is transmitted. If the NDI IE is set to 0 the same TB is transmitted as in a previous transmission (synchronous HARQ is assumed for PUSCH transmissions).
The fifth IE provides a Transmit Power Control (TPC) command for PUSCH transmission power adjustments.
The sixth IE provides a Cyclic Shift (CS) Indicator (CSI), which indicates the CS for a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence used for RS transmission.
The seventh IE indicates whether frequency hopping applies to the PUSCH transmission.
The eighth IE indicates whether the UE should include a DL Channel Quality Indication (CQI) report in the PUSCH.
The ninth IE indicates whether the PUSCH transmission is over a contiguous or non-contiguous BW. The RA IE needs to be re-interpreted in the latter case, but the specifics are not material to the invention and are omitted for brevity.
DCI format 1C is described in Table 2 through a set of IEs for a DL operating BW of NRRDL=6/25/50/100 RBs.
TABLE 2DCI Format 1C IEs for Scheduling PCH,RACH Response, or SIB1 Transmissions.Information ElementNumber of BitsCommentRB Gap0/0/1/1Gap ConfigurationResource Allocation3/7/7/9Restricted Assignment of DL RBsMCS 5Up to 32 MCS LevelsCRC (RNTI)16RNTI for RA or SI or PI masksthe CRCTOTAL24/28/29/31
The first IE indicates the gap value Ngap with Ngap=Ngap,1 or Ngap=Ngap,2. Ngap,1 and Ngap,2 are integer values that depend on the system BW and, for brevity, they are not further described as they are not relevant to the invention. It is assumed that for NRRDL<50 RBs, it is always Ngap=Ngap,1 and the RB Gap IE is only used for NRRDL≧50 RBs.
The second IE provides the resource allocation for PDSCH RBs using ┌ log2(└NVRB,gap1DL/NRBstep┘·(└NVB,gap2DL/NRBstep┘+1)/2)┐
a) NRBstep=2 for 6≦NRBDL<50 and NRBstep=4 for 50≦NRBDL≦110 and
b) NVRB,gap1DL=2·min(Ngap,NRRDL−Ngap) for Ngap=Ngap,1 and NVRR,gap2DL=└NRBDL/2Ngap┘·2Ngap for Ngap=Ngap,2.
Because NVRD,gap2DL≧NVRD,gap2DL, the number of RA bits are reserved assuming NVRB,gap1DL. The RA specifies:
a) the starting RB, RBstart, in steps of NRBstep RBs, with RBstart=0,NRBstep,2NRBstep, . . . , (└NVRBDL/NRBstep┘−1)NRBstep, and
b) the length, LCRB, in virtually contiguous RBs, with LCRDs=NRBstep·2NRBstep, . . . , └NVRBDL/NRBstep┘NRBstep.
An important metric for communication quality that a UE experiences is user plane latency (also known as transport delay), which is defined as the one-way transit time between a Service Data Unit (SDU) packet being available at the Internet Protocol (IP) layer at the UE (or NodeB) and the availability of this packet (Protocol Data Unit, or PDU) at IP layer at the NodeB (or UE). User plane packet delay includes a delay introduced by associated protocols and control signaling for a UE that has synchronized with the NodeB and is in the active state. Advanced communication systems aim to achieve user plane latency less than 10 msec in unloaded conditions (single UE with a single data stream) for small IP packets.
A UL synchronized UE can request a PUSCH transmission by sending a Scheduling Request (SR) on the PUCCH. As the SR transmission is over 1 sub-frame, the shortest SR transmission period is 1 sub-frame. However, to avoid unnecessary SR transmissions, the shortest SR transmission period may also depend on the time period from a time a UE initiates an SR transmission until a time the UE receives an UL SA, at which point the UE knows that the SR was received by the NodeB. The end-to-end process consists of the following steps:
a) UE transmits a SR over 1 sub-frame (1 msec); and
b) NodeB receives the SR, generates and transmits an UL SA, and the UE receives and decodes the UL SA (4 msec).
Assuming that the shortest possible SR transmission period to avoid multiple SR transmissions for the same purpose is 5 msec, the user plane latency is 11.5 msec since the following delays should be included in addition to the previous delays:
a) Average delay to next SR transmission opportunity—2.5 msec (for SR transmission period of 5 msec);
b) UE processing delay of UL SA—if the UL SA is received in sub-frame n, PUSCH transmission is in sub-frame n+4, giving 3 msec for UE processing time; and
c) PUSCH transmission from UE over 1 sub-frame—(1 msec).
Table 3 summarizes the previously described delays.
TABLE 3User Plane Latency for SR Transmission Period of 5 msec.DescriptionDelay (msec)UE: Average Delay for next SR Opportunity2.5UE: SR Transmission (1 sub-frame)1NodeB: SR Reception, SA generation and transmission4(3 sub-frames)UE: SA reception, PUSCH generation and transmission4(4 sub-frames)TOTAL11.5
One approach to reduce user plane latency is to reduce the SR transmission period. For example, reducing the SR transmission period to 1 msec would reduce the user plane latency to 9.5 msec. However, as previously mentioned, the shortest SR transmission period until a UE can know whether its SR was correctly received by the NodeB is 5 msec. Also, a shorter SR transmission period, such as 1 msec or 2 msec, increases the PUCCH overhead as a unique SR resource should be assigned to each UE to avoid potential SR collisions. Also, as the SR transmission period is assigned to a UE through Radio Resource Control (RRC) signaling at connection setup, its fast adaptation is not possible. RRC signaling or Medium Access Control (MAC) signaling will be referred to as higher layer signaling to differentiate such signaling from PDCCH signaling which is through the physical layer.
Another approach to reduce user plane latency would be to reduce the UE and NodeB processing delays. However, this is associated with substantially higher implementation cost, which is not desirable.
Another approach to reduce user plane latency is to have Contention-Based (CB) PUSCH transmissions which are generated without prior SR transmission and without a UE-specific UL SA. For CB-PUSCH transmissions, UEs need to be informed by the NodeB of at least a set of available UL RBs. One way to achieve this is through a DCI format with CRC scrambled by a CB-RNTI. A UE needs to know its CB-RNTI in advance and multiple UEs may share the same CB-RNTI. Therefore, collisions may occur as multiple UEs, sharing the same CB-RNTI, may attempt CB-PUSCH transmissions in the same RBs and in the same sub-frame. However, CB-PUSCH transmissions can substantially reduce user plane latency as the delays due to SR transmission by the UE and SR processing and UL SA generation and transmission by the NodeB are avoided. As the NodeB does not know which UE, if any, may have CB-PUSCH transmission, a UE can add its C-RNTI to its MAC PDU. The delay components for CB-PUSCH transmission are summarized in Table 4. Over 50% reduction relative to SR-based PUSCH transmission is achieved (for 5 msec SR transmission period).
TABLE 4User Plane Latency for SR Transmission Period of 5 msec.DescriptionDelay (msec)UE: Average Delay for beginning of next sub-frame0.5UE: PDCCH with CB-RNTI Reception5UE: PUSCH generation and transmission (4 sub-frames)TOTAL5.5
Therefore, there is a need to define the IEs for a DCI format supporting CB-PUSCH transmissions for various operating BWs.
There is another need to define methods for assigning RBs to CB-PUSCH transmissions using a DCI format.
Further, there is need to support CB-PUSCH transmissions while minimizing the associated signaling overhead and UE power consumption.